1. Field of the Invention
The present invention relates generally to a positioning device and a positioning method for positioning a drive unit of which a torque constant fluctuates depending on a position, and more particularly to a positioning device and a positioning method for reducing torque fluctuations of a driving unit.
2. Description of the Related Art
A DC motor such as a voice coil motor etc is widely utilized as a driving unit of a positioning device. A positioning accuracy on the order of 1 micron or under is requested of such a positioning device.
FIG. 7 is an explanatory diagram showing a prior art positioning mechanism. FIG. 8 is a control block diagram in the prior art. FIG. 9 is an explanatory diagram showing an operation in the prior art. FIG. 10 is a diagram showing a torque characteristic of a DC motor. FIG. 11 is an explanatory diagram showing a positioning operation of a servo control system. FIG. 12 is an explanatory diagram showing a drive current value.
As illustrated in FIG. 7, an arm 90 rotates about a rotary shaft 93. A coil 91 is wound on a rear end of the arm 90. A pair of magnets 92 are provided in a face-to-face relationship with the coil 91. In this type of rotary actuator, an electric current flows through the coil 91, thereby rotating the arm 90.
Referring to FIG. 10, the solid line indicates a torque measured value measured in each position of the arm 90 by flowing a constant current through the coil 91, and a one-dotted chain line indicates a torque design value under the same conditions. As shown in FIG. 10, the torque measured value exhibits such a characteristic that this value decreases in positions of marginal areas on both sides of an arm movable range.
The reason therefor is that a magnetic flux changes when the coil of the arm 90 is, as illustrated in FIG. 9A, disposed in the central position of the magnet and when one side of the coil of the arm 90, as illustrated in FIG. 9B, becomes proximal to the edge of the magnet. Namely, as shown in FIG. 9B, in a limit position in the arm movable range, one side of the coil is disposed at the edge of the magnet 92, while the other side of the coil is disposed at the center of the magnet 92. At the edge of the magnet 92, a leakage magnetic flux of the magnet 92 increases, and, at the central position of the magnet 92, the magnetic flux decreases due to mutual interference between the upper and lower magnets.
This torque fluctuation might induce a decline of the positioning accuracy and a prolonged positioning time in the positions at both edges of the magnet. As shown in FIG. 12, for keeping this torque constant in all movable range, it is required that the drive current applied to the coil 91 be increased in the areas where the torque measured value decreases.
FIG. 8 is a control block diagram showing a servo control system. As shown in FIG. 8, the servo control system obtains a positional error between a command position and a present position. Then, the positional error is multiplied by the control parameters PID, thus creating a command value. The control parameters are a proportion P, an integration I and a differential D. This command value is given to a motor. The command value is converted into a position by use of a mechanical transfer function K of the motor.
In the positioning operation of the servo control system, as indicated by a solid line in FIG. 11, the drive current flows so that a deceleration is performed after implementing an acceleration. With this operation, the position of the actuator converges in the command position, as shown by the solid line in FIG. 11. In the servo control system, the command value changes corresponding to the positional error, and hence a drive current value increases as indicated by a dotted line in FIG. 11 in the area where the torque becomes deficient. Therefore, when using the servo control system, the drive current value is automatically controlled so as to make the torque constant.
There arise, however, the following problems inherent in the prior art.
First, when servo controlling so as to obtain the constant torque in all movable range, a difference on the order of several hundreds mA generates between the value of the drive current applied to the coil in the central area and the drive current in the marginal areas on both side of the movable range. In a voltage/current converting unit (driver) for converting the command value into the drive current, a maximum output current value of the driver is determined corresponding to a maximum drive current in the marginal areas on both side of the movable range, in which the current value is needed most. In the prior art, a gain of the driver is fixed. Therefore, it is required that the current range of the command value be enlarged. According to this method, however, there might increase a quantization error in the command value of a control unit (processor) for calculating the current command value. Therefore, a minute positioning accuracy declines. Particularly, there is brought about a decline of the stepwise positioning accuracy as minute as approximately 1 micron.
Second, the maximum drive current value required in the central area is 20 mA. Therefore, when the quantization error in the command value is large, the S/N ratio of the drive current might deteriorate in the central area.
Third, it can be considered that the maximum current value of the driver is decreased in order to reduce the quantization error in the command value of the control unit. If done so, as indicated by the dotted line in FIG. 11, it follows that the drive current is limited in the marginal areas on both sides of the movable range. The drive current thereby takes a saw-tooth configuration, resulting in vibrations of the actuator.
It is a primary object of the present invention to provide a positioning device and a positioning method capable of minimizing a torque fluctuation in a whole movable range of a driving unit.
It is another object of the present invention to provide a positioning device and a positioning method capable of obtaining a proper quantization error even when a maximum current value in the movable range might change.
It is a further object of the present invention to provide a positioning device and a positioning method capable of preventing a decline of a positioning accuracy even when moved at a minute distance within the whole movable range.
To accomplish the above objects, according to one aspect of the present invention, a positioning device comprises a position detector for detecting a present position of the drive unit, a control circuit for generating a digital drive command value corresponding to a positional error between a command value and the present position, a D/A converter for converting the digital drive command value into an analog voltage, and a driver, having a plurality of gains, for converting the analog voltage into the analog drive current of the drive unit with a selected gain. The control circuit select the gain of the driver in accordance with the present position.
A positioning method for positioning a drive unit in a command position, comprises a step of detecting a present position of the drive unit, a step of calculating a digital drive command value corresponding to a positional error between the command position and the present position, a step of selecting a drive gain in accordance with the present position, a step of D/A converting the digital command value into an analog voltage, and a step of converting the analog voltage into an analog drive current of the drive unit with said selected drive gain.
According to the present invention, the gain of the driver (voltage/current converting amplifier) is controlled corresponding to the position of the drive unit, thereby controlling a maximum current value. With this contrivance, there are provided the driver (voltage/current converting amplifier) having the plurality of gains, and the control circuit for selecting the gain corresponding to the detected position.
Thus, when the gain of the voltage/current converting amplifier is made variable and controlled corresponding to the position, there is no necessity for covering the maximum current range with the digital command value. Therefore, the proper quantization error is obtained in the entire movable range, and the maximum current value needed in that position is obtained.
According to another aspect of the present invention, the drive unit is constructed of a DC actuator having a magnet and a coil, and the control circuit selects the gain of the voltage/current converting amplifier so that a first selected gain at a central area of a movable range of the DC actuator differs from a second selected gain of marginal areas on both sides of the movable range.
According to still another aspect of the present invention, the control circuit has a plurality of control parameters for calculating the digital drive command value from the positional error, and selects the control parameter corresponding to the selected gain.
According to a further aspect of the present invention, the voltage/current converting amplifier includes an input gain setting circuit for setting an input gain of the inputted analog voltage between a plurality of input gains and an amplifier for converting into an analog current the analog voltage given the input gain from the gain setting unit.
According to a still further aspect of the present invention, the control circuit is constructed of a processor.
Other features and advantages of the present invention will become readily apparent from the following description taken in conjunction with the accompanying drawings.